Problem with leaking bathtub sealing

Hello everyone, this is my first post.
We have a house with very limited space and
couple of years ago I installed bathtub.
Here is the picture how it is placed:

So, it is a little bit non standard installation.
We could not use a tub with borders on left, right and long sides,
which are for wall-attached installations, obviously.
So, this is a "freestanding" tub without borders on any side.
I bought separate L shape profiles to be attached to places
where borders should be.
And it seems worked.

Now, we have leaks underneath (there is a non-finished basement,
so water just goes down). Not too much, probably less then a litter
when somebody takes shower.
I tested it with shower on - no leaks,
then I realized that when somebody gets into tub, under his weight
tub borders goes down a little and there is millimetre or a few
is a gap, where water from shower gets in.

I can see, that silicon I used (GE Silicon II) is pealed off from the tub.

Is it supposed to bond to tub? It is acrylic fiberglass tub.
What should I use - same silicon or something better?
I think, before doing resealing, I should put lot of weight in the tub
and fill with water, so tub will be "in down position".

Interesting, that it peels off even in the place where tub should not have any
movement:

Perhaps, I have not cleaned area to be sealed properly.
Any suggestions how to do it in a proper way?

Not sure you'll ever get that sealed properly. WHen using a drop-in tub as a shower, you need an add-on tiling flange. If one of those is used and installed properly, the lip will keep water inside of the tub. At least along one side, it appears your tile goes up against the edge of the tub and the only seal is caulk.

If you have a removable panel, you may be able to push some mortar underneath the tub to stabilize it so it can't deflect. That would help, then caulk may last longer. You may want to consider adding a full circle shower rod, which should keep most water off the sidewalls. If the caulked areas have any gap, filling it first with foam backer rod, then caulking will also extend the life of the caulk. This forces the caulk into an hour-glass shape so it has a narrower section that flexes better than a block of it. It is less likely to tear off one edge when done that way, plus, you end up using less caulk. The backer rod comes in all sorts of diameters, but you may need to search to find it in the size you need. The big box stores only tend to carry the big stuff, not the smaller diameter stuff you'd likely need.

The problem is that the tub moves. A tub like that should be supported in a mortar bed or otherwise secured to that it can never move. Continued replacement of the caulk is a stopgap measure to fixing the root of the problem.

Not sure you'll ever get that sealed properly. WHen using a drop-in tub as a shower, you need an add-on tiling flange. If one of those is used and installed properly, the lip will keep water inside of the tub. At least along one side, it appears your tile goes up against the edge of the tub and the only seal is caulk.

If you have a removable panel, you may be able to push some mortar underneath the tub to stabilize it so it can't deflect. That would help, then caulk may last longer. You may want to consider adding a full circle shower rod, which should keep most water off the sidewalls. If the caulked areas have any gap, filling it first with foam backer rod, then caulking will also extend the life of the caulk. This forces the caulk into an hour-glass shape so it has a narrower section that flexes better than a block of it. It is less likely to tear off one edge when done that way, plus, you end up using less caulk. The backer rod comes in all sorts of diameters, but you may need to search to find it in the size you need. The big box stores only tend to carry the big stuff, not the smaller diameter stuff you'd likely need.

Click to expand...

Yes, there is L shaped plastic profile underneath tile which is glued to the tub with silicone.
Tub itself lays on wooden support from 2x4 with stubs but no mortar bed.

The problem is that the tub moves. A tub like that should be supported in a mortar bed or otherwise secured to that it can never move. Continued replacement of the caulk is a stopgap measure to fixing the root of the problem.

Where are the areas you took the pictures of. I cannot see anywhere in the picture of the bathtub where that first area could be.

Click to expand...

On the left side there is a shampoo in white package - right under that package, picture left corner of the tiled "shelf" where shapoo stays, could be seeing.
It is where most leaks goes - on left side of the tub, if looking at the top picture.
Right side quit good.

Would it be ok, if I use epoxy first to let it fill spaces where tub connects with tile?
Instead of fixing structure with mortar?
And then on top, cover with silicone.

Would it be ok, if I use epoxy first to let it fill spaces where tub connects with tile?
Instead of fixing structure with mortar?
And then on top, cover with silicone.

Click to expand...

Industry standards call for caulking any changes of plane or materials...mortar has essentially no flex and epoxy might be worse - you could crack tile, and between the wall, tub, there's likely some either thermal movement caused by uneven expansion/contraction, or flexural movement.

Note, a space used as a shower should be waterproof before the tile is installed...so, caulk and grout or even the tiles, while necessary as a wear and appearance surface, shouldn't be necessary to keep a shower waterproof.

If the gap is large, you should look for foam caulk backer rod...this comes in many different diameters - the correct size should be slightly compressed when placed into the crack. This forces the caulk into an hour-glass shape, which is much less likely to fail than a big blob forced into a crack. The thin part caused by the round backer rod gives the caulk a place to flex rather than a big blob that might get pulled off one of the bonding surfaces.

I don't know if this will help but you might want to read this thread. I used that Lexel sealant that was suggested and I have not had a leak since...and it's been 3 1/2 years now!! It is very stickly stuff! Good luck!